Wednesday, June 29, 2016

Figure 1. Distribution of coexisting friarbirds (Philemon) and brown orioles (Oriolus) with the phylogeny of brown orioles oriented to match the phylogeny of friarbirds (see also electronic supplementary material, figures S3–S5 for additional phylogenetic information). Species names are coloured according to island distributions. To the right of taxon names are indicated distributions (Au, Australia; NG , New Guinea) followed by a number which indicates coexistence. For example, Philemon brassi (NG, 1) occurs in New Guinea and is sympatric with Oriolus szalayi (NG, 1,5,6,9).

Timescales (in million years) are indicated below the phylogenies.

Illustrations from Handbook of the Birds of the World [del Hoyo. et. al, 2008]. Asterisks at nodes indicate posterior probabilities of 0.99–1.00. DOI: 10.1098/rspb.2016.0409

Abstract

Observations by Alfred Wallace and Jared Diamond of plumage similarities between co-occurring orioles (Oriolus) and friarbirds (Philemon) in the Malay archipelago led them to conclude that the former represent visual mimics of the latter. Here, we use molecular phylogenies and plumage reflectance measurements to test several key predictions of the mimicry hypothesis. We show that friarbirds originated before brown orioles, that the two groups did not co-speciate, although there is one plausible instance of co-speciation among species on the neighbouring Moluccan islands of Buru and Seram. Furthermore, we show that greater size disparity between model and mimic and a longer history of co-occurrence have resulted in a stronger plumage similarity (mimicry). This suggests that resemblance between orioles and friarbirds represents mimicry and that colonization of islands by brown orioles has been facilitated by their ability to mimic the aggressive friarbirds.

We present a detailed analysis of a classic example of visual mimicry in birds in Australo-Papua and the surroundingarchipelagos, using nearly complete dated molecular sub-species-level phylogenies, and drawing on several lines ofevidence to determine how brown orioles (the mimics) evolvedin relation to friarbirds (the model). Consistent with the mimicry hypothesis, our analyses show that friarbirds evolved before brown orioles and that they speciated and dispersed largely independently of each other in Australo-Papua and the surrounding archipelagos. In the depauperate island environments, improved mimicry probably facilitated the persistence of brown orioles. Our data also show that orioles tend to bettermatch the colour of sympatric friarbirds than allopatric friarbirds. Finally, a longer history of co-occurrence and a larger size difference between model and mimic lead to better mimicry, ultimately allowing for the existence of both friarbirds and orioles on Wallacean islands.